1. Field of the Invention
The present invention relates to hydrotreating processes to efficiently reduce the sulfur content of hydrocarbons.
2. Description of Related Art
The discharge into the atmosphere of sulfur compounds during processing and end-use of the petroleum products derived from sulfur-containing sour crude oil poses health and environmental problems. Stringent reduced-sulfur specifications applicable to transportation and other fuel products have impacted the refining industry, and it is necessary for refiners to make capital investments to greatly reduce the sulfur content in gas oils to 10 parts per million by weight (ppmw) or less. In the industrialized nations such as the United States, Japan and the countries of the European Union, refineries have already been required to produce environmentally clean transportation fuels. For instance, in 2007 the United States Environmental Protection Agency required the sulfur content of highway diesel fuel to be reduced 97%, from 500 ppmw (low sulfur diesel) to 15 ppmw (ultra-low sulfur diesel). The European Union has enacted even more stringent standards, requiring diesel and gasoline fuels sold in 2009 to contain less than 10 ppmw of sulfur. Other countries are following in the footsteps of the United States and the European Union and are moving forward with regulations that will require refineries to produce transportation fuels with ultra-low sulfur levels.
To keep pace with recent trends toward production of ultra-low sulfur fuels, refiners must choose among the processes or crude oils that provide flexibility that ensures future specifications are met with minimum additional capital investment, in many instances by utilizing existing equipment. Conventional technologies such as hydrocracking and two-stage hydrotreating offer solutions to refiners for the production of clean transportation fuels. These technologies are available and can be applied as new grassroots production facilities are constructed. However, many existing hydroprocessing facilities, such as those using relatively low pressure hydrotreaters, represent a substantial prior investment and were constructed before these more stringent sulfur reduction requirements were enacted. It is very difficult to upgrade existing hydrotreating reactors in these facilities because of the comparatively more severe operational requirements (i.e., higher temperature and pressure) to obtain clean fuel production. Available retrofitting options for refiners include elevation of the hydrogen partial pressure by increasing the recycle gas quality, utilization of more active catalyst compositions, installation of improved reactor components to enhance liquid-solid contact, the increase of reactor volume, and the increase of the feedstock quality.
There are many hydrotreating units installed worldwide producing transportation fuels containing 500-3000 ppmw sulfur. These units were designed for, and are being operated at, relatively mild conditions (i.e., low hydrogen partial pressures of 30 kilograms per square centimeter for straight run gas oils boiling in the range of from 180° C. to 370° C.).
With the increasing prevalence of more stringent environmental sulfur specifications in transportation fuels mentioned above, the maximum allowable sulfur levels are being reduced to no greater than 15 ppmw, and in some cases no greater than 10 ppmw. This ultra-low level of sulfur in the end product typically requires either construction of new high pressure hydrotreating units, or a substantial retrofitting of existing facilities, e.g., by incorporating gas purification systems, reengineering the internal configuration and components of reactors, and/or deployment of more active catalyst compositions.
Sulfur-containing compounds that are typically present in hydrocarbon fuels include aliphatic molecules such as sulfides, disulfides and mercaptans as well as aromatic molecules such as thiophene, benzothiophene and its long chain alkylated derivatives, and dibenzothiophene and its alkyl derivatives such as 4,6-dimethyl-dibenzothiophene.
Aliphatic sulfur-containing compounds are more easily desulfurized (labile) using mild hydrodesulfurization methods. However, certain highly branched aromatic molecules can sterically hinder the sulfur atom removal and are moderately more difficult to desulfurize (refractory) using mild hydrodesulfurization methods.
Among the sulfur-containing aromatic compounds, thiophenes and benzothiophenes are relatively easy to hydrodesulfurize. The addition of alkyl groups to the ring compounds increases the difficulty of hydrodesulfurization. Dibenzothiophenes resulting from addition of another ring to the benzothiophene family are even more difficult to desulfurize, and the difficulty varies greatly according to their alkyl substitution, with di-beta substitution being the most difficult to desulfurize, thus justifying their “refractory” appellation. These beta substituents hinder exposure of the heteroatom to the active site on the catalyst.
The economical removal of refractory sulfur-containing compounds is therefore exceedingly difficult to achieve, and accordingly removal of sulfur-containing compounds in hydrocarbon fuels to an ultra-low sulfur level is very costly by current hydrotreating techniques. When previous reaulations permitted sulfur levels up to 500 ppmw, there was little need or incentive to desulfurize beyond the capabilities of conventional hydrodesulfurization, and hence the refractory sulfur-containing compounds were not targeted. However, in order to meet the more stringent sulfur specifications, these refractory sulfur-containing compounds must be substantially removed from hydrocarbon fuels streams.
The relative reactivity of thiols and sulfides are much higher than those of aromatic sulfur compounds, as indicated in a study published in Song, Chunshan, “An Overview of New Approaches to Deep Desulfurization for Ultra-Clean Gasoline, Diesel Fuel and Jet Fuel,” Catalysis Today, 86 (2003), pp. 211-263. Mercaptans/thiols and sulfides are much more reactive than the aromatic sulfur compounds. It should be noted that non-thiophenic sulfides such as paraffinic and/or naphthenic are present in diesel range hydrocarbons as can be seen from the relative reactivity chart shown in FIG. 1.
Studies have been conducted related to increasing the relative reactivity of sterically hindered sulfur-containing hydrocarbons. In particular, isomerization of 4,6-dimethyl-dibenzothiophene into methyl-migrated isomers and tri- or tetramethyl-dibenzothiophenes was studied. Seelsoda et al., “Hydrodesulfurization Pathway of 4,6-Dimethyldibenzothiophene Through Isomerization over Y-Zeolite Containing CoMo/Al2O3 Catalyst”, Energy & Fuels, 1996, 10, 1078-1082, and Isoda et al., “Changes in Desulfurization Reactivity of 4,6-Dimethyldibenzothiophene by Skeletal Isomerization Using a Ni-Supported Y-Type Zeolite,” Energy & Fuels, 2000, 14, 585-590.
McConnachie, et al. U.S. Pat. No. 7,731,838 describes a process in which a diesel boiling range feedstream which includes nitrogen-containing compounds and hindered dibenzothiophenes is subjected to removal of nitroun-containing compounds by contacting the feedstream with a sulfuric acid solution, contacting the reduced nitrogen stream with a solid acid catalyst to isomerize certain sulfur-containing molecules; and hydrotreating the isomerized effluent. However, the described process in McConnachie, et al. prefers that the entire feed is both subjected to the removal of nitrogen-containing compounds by contacting the feedstream with a sulfuric acid solution, and contacted with the costly and sensitive (i.e., easily poisoned) solid acid isomerization catalyst. This requires that the nitrogen-removal apparatus have a capacity suitable for the entire feedstream, and impart excessive demand on this isomerization catalyst and its reactor capacity.
McVicker, et al. U.S. Pat. No. 5,897,768 teaches a desulfurization process in which an entire feedstream is hydrotreated using conventional catalysts. The partially hydrotreated effluent is fractionated, whereby sterically hindered sulfur-containing hydrocarbons are removed with the bottoms stream. The bottoms stream is passed to a reactor containing isomerization catalyst. The effluent from the reactor containing isomerization catalyst is returned to the hydrotreating reactor. However, in McVicker, et al., the entire initial feed is passed through the hydrotreating reactor, including refractory sulfur-containing hydrocarbons which are likely not desulfurized in this initial pass-through, thus decreasing the overall process efficiency.
None of the above-mentioned references describe processes for desulfurization of including specific sub-processes and apparatus for targeting different organosulfur compounds. With the steady increase in demand for hydrocarbon fuels having an ultra-low sulfur level, a need exists for an efficient and effective process and apparatus for desulfurization.